The present invention relates to capacitors which are capable of exhibiting high energy capacitance and high current density discharge over relatively extended time periods ranging from a few seconds to minutes. Such "supercapacitors" are particularly useful for delivering high levels of electrical current to utilization devices in a much shorter time than required by battery systems alone. The invention is directed in particular to a supercapacitor structure, and method of making the same, which incorporates the basic capacitor elements into a unitary, flexible structure which may be sized and shaped as desired to be most compatible with utilization devices while providing advantageously high energy capacities and current densities. The invention further takes advantage of available polymeric rechargeable battery technology in order to provide hybrid supercapacitor systems which are capable of retaining the power and dimensioning versatility of the novel supercapacitor structures.
In large measure, available supercapacitors are of the double layer type in which a pair of electrodes, typically comprising particulate activated carbon, are separated by a microporous, electron-insulating, ion-conducting sheet element comprising a uniformly-dispersed electrolyte component. The structure of the typical supercapacitor further comprises electrically-conductive current collector elements in intimate contact with the respective electrodes. Common among the structural variants of such prior supercapacitor devices are means, such as compressive arrangements, which maintain the essential close physical contact between elements in order to ensure low internal electrical resistance. An example of a capacitor of this type may be seen in U.S. Pat. No. 3,536,936 where the considerable compacting pressure required to reduce to usable levels the internal electrical resistance of the carbon particle electrode composition, as well as of the electrode/collector interface, creates severe difficulties in the fabrication of the capacitor cell.
Attempts have been made to reduce the internal electrical resistance in supercapacitor electrodes by means other than directly-applied physical pressure, notably through some manner of unifying the particulate carbon electrode composition and conductive collectors. A process of high-temperature sintering of the elements to achieve this end is described in U.S. Pat. No. 5,115,378, yet, as is apparent there, the extensive processing steps and high energy consumption lead to economic undesirability of this approach. Further limiting the general acceptance of the process is the intractability of the resulting solid and unyielding preformed capacitor body which cannot be readily shaped to conform to spacial requirements of varying utilization devices.
Other means for minimizing the internal resistance of supercapacitor structures have, for example, attempted to combine pyrolyzed aerogel, carbon foam electrodes with high-temperature soldering of conductive collector elements, as described in U.S. Pat. No. 5,260,855. Such approaches have realized limited success, however, due to the extensive processing and high energy and time consumption required, in addition to the lack of manipulability of the resulting devices.
Overcoming the limitations of prior supercapacitor structures and fabrication procedures, the present invention provides, in particular, means for readily preparing flexible, low resistance supercapacitor structures under economical ambient conditions. Such simple fabrication procedures enable the expanded use of these devices in a wide variety of configurations and applications, including combinations with integrated rechargeable battery energy sources of compatible composition and structure.